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POTABLE WATERS                                     7

excess of ammonium molybdate solution, heated to about 40° and stirred,
and examined for yellow turbidity or precipitate.

8. Organic Matter.—Indications of the presence of organic matter
in a water are obtained from the blackening of the dry residue when heated,
provided the amount present is marked. For quantitative determination,
recourse is had to the indirect method of oxidising with potassium per-
manganate and calculating the oxygen necessary for the combustion of
the organic matter.

The methods most commonly used are the following :

(a)  KUBEL'S METHOD, which requires :

(1)  Distilled water free from organic matter, obtained by redistilling
water with a little permanganate.

(2)  Pure dilute sulphuric acid, 1:3.

(3)  N/ioo-potassium permanganate solution, containing 0-3163 gram
of KMn04 per litre.

(4)  N/ioo-oxalic acid, containing 0-63 gram of H2C204, 2H20 per litre.
'     The last two solutions should correspond volume with volume.   To
check this, 20 c.c. of the oxalic acid solution are heated nearly to boiling
with 5 c.c. of the sulphuric acid and then titrated with the permanganate
until a pink coloration persists :   20 c.c. should be required.

To 100 c.c. of the water are added 5 c.c. of the sulphuric acid and then
10 or more c.c. of the permanganate, so that the liquid remains coloured
even after boiling. The liquid is boiled for 5 minutes, a volume of the
oxalic acid equal to that of the permanganate taken being then added.
The solution, which is then colourless, is titrated in the hot with perman-

The difference between the total volume of permanganate used and
that necessary for the oxidation of the oxalic acid added (in other words
the volume of permanganate used in the final titration) gives the volume
of permanganate consumed in the oxidation of the organic matter. Since
i c.c. of N/ioo-permanganate corresponds with 0-00008 gram of oxygen,
the number of c.c. of permanganate consumed must be multiplied by 0-08
to obtain the grams of oxygen used up in oxidising the organic matter in
100 litres of the water. On the assumption that I part of permanganate
oxidises very nearly 5 parts of organic matter, the amount of the latter
per 100 litres of water is sometimes calculated by multiplying the number
of c.c. of permanganate used up by 1-58.

(b)  SCHULZE AND TROMMSDORFF's METHOD.   This requires the same
reagents as the preceding method, together with pure sodium hydroxide
solution (1:2).

One hundred c.c. of the water are boiled for 10-15 minutes with 0-5 c.c.
of the caustic soda solution and 10 c.c. of the permanganate, allowed to
cool to about 60°, mixed with 5,c.c. of the sulphuric acid and 10 c.c. of the
oxalic acid solution, and titrated with the permanganate. The oxygen
consumed and the organic matter present are then calculated as described
under (a}.

With either of these methods it is always necessary to make a blankhe water, either as